Spot the Frippet: grit.

 Grit is the stuff which causes friction.

Sometimes that's a good thing:

photo of sandpaper by Simon Eugster 

photo of gritting lorry by Sebastiandoe5

A grinding stone can be made of the kind of sandstone called grit, too:

millstone grit formation, photo by Ceri Thomas

Sometimes grit is a nuisance (though even then....): 

pearl in oyster, photo by Manfred Heyde

Sometimes it's even heroic, if the kind of grit you're talking about is courage or determination:

Mostly, luckily, the nearest we have to get to that is having to grit our teeth from time to time.

Hope today's grits are all the good kind:

photo by Ernesto Andrade

(Though opinions differ widely and passionately about whether that stuff is a good kind.)

Spot the Frippet: grit. This word is grēot in Old English.